In the conventional jack, the hydraulic cylinder, and pulling arm on the end of the extension rod are usually installed directly under the lifting arm. When the extension rod drives the pulling rod, the pivotally connected ends of the extension rod and the pulling rod are on a horizontal displacing point. The hydraulic cylinder can only be fixed in place at its rear end; therefore, when the hydraulic cylinder applys pressure to drive the extension rod, the load on the lifting arm causes the joint portions on both front and rear ends of the hydraulic cylinder to become disengaged as a result of a change in the relative angle between the pulling rod and and the elbow to generate a one-way vertical component of force. Consequently, the hydraulic cylinder is liable to leak.